The radio-frequency acceleration cavity is a cavity made of metal devised to resonate a radio-frequency wave of a particular frequency so as to accelerate charged particles with efficiency by using the radio-frequency electric field, and is used in a charged particle accelerator such as a synchrotron.
For the radiofrequency cavity, since generation of the radio-frequency wave produces heat, metal materials are suitable that are high in thermal conductivity and low in electric resistance. Conventionally, copper has been used as raw materials of such a radio-frequency acceleration cavity. However, since the calorific value increases as the acceleration electric field increases, radio-frequency acceleration cavities made of copper materials have limitations in improvements in performance. Therefore, in recent years, superconducting cavities have been proposed and used. Then, niobium materials (in the present invention, the “niobium materials” include niobium alone and alloys of niobium and another metal (for example, copper)) are used because niobium causes superconducting transition at the highest absolute temperature as an elemental metal and has the advantage of being relatively ease to process as a metal, and currently, the radio-frequency acceleration cavity made of niobium materials is being put into practical use.
FIG. 9 is to explain the principle of accelerating the velocity of a charged particle in the radio-frequency acceleration cavity. In the case of assuming that the length of a pipe is d, in the radio-frequency wave the frequency is f, the wavelength is λ, and the cycle is T, and the velocity of a charged particle is v, when time t=d/v required to pass through a single pipe is a half of the cycle T, the charged particle is accelerated in each of coupled pipes. Herein, since v=fλ and T/2=t=d/v=d/fλ=dT/λ, the length of a single pipe is designed so that d=λ/2. By this means, whenever the number of pipes to couple is increased, the charged particle obtains energy from each pipe, and it is thereby possible to accelerate the velocity of the charged particle cumulatively.
Niobium is relatively soft ash gray metal (transition metal), and has body-centered cubic lattice structure that is stable crystal structure at room temperature under normal pressure, and the specific gravity of 8.56. In air, the oxide layer is formed and has corrosion resistance and acid resistance. Niobium causes superconducting transition at 9.2K (under normal pressure) that is the highest absolute temperature as an elemental metal.
A large amount of niobium thin plates with thicknesses of the order of several millimeters is required to manufacture a superconducting radio-frequency acceleration cavity made of niobium materials.
In the conventional technique, as a method of obtaining a niobium thin plate with a thickness of the order of several millimeters, there have been a plasticity processing method of cutting a required amount from a high-purity niobium ingot, and then, performing forging and rolling thereon, and a saw method of slicing a niobium ingot with a diameter of a few tens of centimeters using a band saw machine or the like.